Mattress advice for big hips / back sleeping

Hello. I need help! I can't face doing another trawl of the mattress shops. I had an old mattress that collapsed in the summer. After this I felt I needed firm support so bought a firm foam mattress from ikea that was like sleeping on concrete. After that I decided to spend more money and ordered a John Lewis Natural Collection Cotton 4000 Special Buy medium over Christmas. That was way too hard and I was still in agony and so, after many more hours spent in the showroom and many more bits of advice from various salesmen and women, I exchanged it for the Cotton 3000 medium, and that is also resulting in me waking in agony with mid back / shoulder pain and feels like my hips are sinking in without any support for my lower back curve - it feels even worse!
I have large hips and therefore quite a curve in my back that needs filling (around 6cm). I'm not a huge fan of the feel of memory foam (I don't like feeling stuck in it) or latex (the ones I've laid on in shops have felt too firm). I also get quite hot in bed with some synthetic materials in a mattress (although my pillows and duvet are synthetic). I'm really not sure what to do now. The thought of wasting more hours in show rooms fills me with horror, as I've been kicked out of JL at closing time on a few occasions now... Should I be looking for a cheaper mattress and some sort of topper? Part of me is even tempted to spend more if it would guarantee I would get it right, but I'm not sure it would. Also I might rent out my flat at some point in the future, so a really pricey one might not be the best idea.
I was about to buy the Balmoral from Warren Evans in their recent sale as the reflex foam seemed to push up on my hand when I put it under the curve of my back (something the sales people in JL said I needed to do to test support for my back), but then the WE sales person told me that the Balmoral would feel too hard, so I gave up.
I have a bowed slated base and metal framed king-sized bed. I sleep on my back and side and am around 13.5 stone.
Any help appreciated!
Thanks guys.
L